1. Field of the Invention
Embodiments of the present invention generally relate to cutting inserts for use with wellbore milling tools.
2. Description of the Related Art
Oil and gas wells typically begin by drilling a borehole from the earth's surface to some predetermined depth adjacent a hydrocarbon-bearing formation. After the borehole is drilled to a certain depth, steel tubing or casing is typically inserted in the borehole to form a wellbore. Various completion and production operations occur within the wellbore that require the use of milling tools.
Milling tools can be used to cut out windows or pockets from a tubular for such operations as directional drilling and sidetracking. In addition, mills can remove materials downhole such as pipe, casing, casing liners, tubing, or jammed tools by milling through them. Milling tools have been used for removing a section of existing casing from a wellbore, to provide a perforated production zone at a desired level, to provide cement bonding between a small diameter casing and the adjacent formation, or to remove a loose joint of surface pipe. Also, milling tools can be used for milling or reaming collapsed casing, for removing burrs or other imperfections from windows in the casing system, for placing whipstocks in directional drilling, or for aiding in correcting dented or irregular areas of a tubular. These milling tools have cutting blades or surfaces and are lowered into the well or casing and then rotated in a milling/cutting operation. With certain tools, a suitable drilling fluid is pumped down a central bore of a tool for discharge beneath the cutting blades or surfaces and an upward flow of the discharged fluid in the annulus outside the tool removes from the well cuttings or chips resulting from the cutting operation.
Several different types of inserts currently exist for use on a milling tool. Inserts are typically elements made of very hard material such as tungsten carbide. These inserts are typically welded or bonded to a portion of the mill such as a blade or tip. Therefore, the completed blade portion of the mill comprises three layers of materials including the blade that is usually steel, the bonding material that is usually brass, and the insert that is usually tungsten carbide. Differences in thermal expansion of these three layers can cause delaminating to occur at the bond surfaces or stress cracks in the inserts or blades on the mill that adversely affect the mill performance. Sections of carbide blade that detach from the tool may have to be retrieved from the wellbore at significant costs.
Surfaces on inserts that contact the material being milled include flat planar surfaces, convex surfaces, concave surfaces, or various other geometrical shapes advantageous to the cutting process. Certain of these inserts have surface irregularities, recesses, or indentations that serves as a chipbreaker to break a cutting being produced by an insert to limit the length of the cuttings. The inserts must be positioned on the blade of the mill with the proper cutting angle. Therefore, inserts can be formed with angled surfaces that contact the material being milled by adding material to one side of the insert to form an angled surface. In addition, milling slots in a vertical blade, leaning blades at an angle, or spiraling blades around a mill body can place the inserts at the correct cutting angle while adding further expense to the manufacture of the mill. However, the force during milling that acts on the surface of prior insert designs breaks off the edge formed by the tallest portion since there is minimal material to support the force acting on the edge.
Therefore, there exists a need for an improved apparatus for use in milling operations in a wellbore. There is a further need for an improved and more reliable cutting insert for a tool used in wellbore milling operations.